1. RNA, & Protein Synthesis
7th Grade, Week 4, Day 1
Monday, July 15, 2013

2. The Central Dogma

3. RNA vs. DNA
Ask them if they can point out some of the differences already?

4. Ribonucleic Acid
RNA is required for translation of genetic information stored in DNA into protein products
Transcribed from DNA
Contains ribose sugar instead of deoxyribose
Single stranded instead of double stranded
Uracil instead of
thymine

5. Types of RNA
Precursor mRNA (pre-mRNA): an immature form of messenger RNA that contains introns and exons
Messenger RNA (mRNA): contains only exons that form the code for the sequence of amino acids that makes up a protein
Transfer RNA (tRNA): decodes the message contained in mRNA and allows for the synthesis of proteins
Ribosomal RNA (rRNA): forms part of the structure of ribosome

6. Transcription Making RNA from DNA Occurs in the nucleus
RNA Polymerase synthesizes pre-mRNA from DNA in the 5’  3’ direction
Happens in much the same way that DNA polymerase synthesizes new strands of DNA during replication

7. Transcription
Template (antisense) strand is transcribed, while the other (sense) strand remains inactive
Uracil bonds with Adenosine in place of Thymine
Initial pre-mRNA made contains both exons and introns
So which strand is going to be identical to the new pre-mRNA? The non-template strand (which is why it’s also called the sense strand).

8. 3 Steps of Transcription
Initiation: occurs when the RNA polymerase binds to promoter and forms the transcription bubble
Elongation: the RNA chain is lengthened by the addition of bases in the 5’ to 3’ direction
Termination: RNA polymerase runs into a termination region.

9. Splicing
In order for the pre-mRNA to leave the nucleus and travel to the ribosomes in the cytoplasm for translation, it must be made into mature mRNA
This happens by removing the introns so that only exons are left
An enzyme complex called a spliceosome performs this task

10. Alternative Splicing
Not all exons are always left – sometimes they get spliced out as well
Therefore, the same pre-mRNA can be spliced differently to get different gene products (proteins) and create diversity

11. RNA Processing The cap: is a modified guanine (G)
Addition of a poly-adenosine (poly-A) tail and a 5’ cap before the mRNA can exit the nucleus and move into the cytoplasm
The cap: is a modified guanine (G)
protects the RNA from being degraded by enzymes that degrade RNA from the 5′ end;
serves as an assembly point for the proteins needed to recruit the small subunit of the ribosome to begin translation.
Don’t worry about the un-translated regions.

12. Proteins
DNA acts like a blueprint that determines the structure of every protein made in your body
Every protein is made up of amino acids
There are 20 amino acids:
Essential: must be supplied in the diet
Non – essential: synthesized de novo
We obtain most of our amino acids by digesting proteins taken in with our food.
The digestive process breaks the protein chains down into individual amino acid molecules which are then absorbed by the blood and transported to the individual body cells.

13. Proteins
During protein synthesis, the separate amino acids are reassembled into new chains. Each kind of protein has its own particular sequence of amino acids, which differs from the sequence in every other kind of protein.
Just the way the order of letters in a word give it its own specific form and meaning, it is the order of the amino acids in the chain that determines the protein's structure and function.
The code for ordering the amino acids of a protein is written as a sequence of bases in the DNA in the nucleus.

14. The Genetic Code
Triplet code: codons are made of 3 nucleotide bases which are non-overlapping
The system is redundant – amino acids are encoded by more than one codon
Practice – Translate:
5’ – AUG ACU AAU GCU UAA – 3’
Answer: Met Thr Asn Ala Stop

15. Translation
mRNA is translated into amino acids using the genetic code, which are then assembled into a protein
This process takes place in the cytoplasm on ribosomes
Both mRNA and tRNA are necessary for this process

16. Steps in Translation Ribosomes bind mature mRNA
There are about 32 different tRNA molecules
Each tRNA molecule has an anticodon that is complementary to a codon on mRNA coding for a certain amino acid (so most amino acids have more than one tRNA that will code for them)
The tRNA will then retrieve that amino acid and bring it to the ribosome for protein assembly

17. Ribosomes Made up of rRNA
Composed of two subunits – one large and one small
Ribosomes can be free in the cytosol or membrane bound to the endoplasmic reticulum (ER) called the "rough ER“
Where the process of protein assembly is carried out

18. Amino acid
U A C
tRNA molecule
A transfer RNA molecule arrives.
It brings an amino acid to the first three bases (codon) on the mRNA.
anticodon
The three unpaired bases (anticodon) on the tRNA link up with the codon.
A U G G G C U U A A A G C A G U G C A C G U U

19. U A C
C C G
Another tRNA molecule comes into place, bringing a second amino acid.
Its anticodon links up with the second codon on the mRNA.
A U G G G C U U A A A G C A G U G C A C G U U

20. U A C
C C G
Peptide bond
A peptide bond forms between the two amino acids.
A U G G G C U U A A A G C A G U G C A C G U U

21. U A C
C C G
The first tRNA molecule releases its amino acid and moves off into the cytoplasm.
A U G G G C U U A A A G C A G U G C A C G U U

22. C C G
The ribosome moves along the mRNA to the next codon.
A U G G G C U U A A A G C A G U G C A C G U U

23. C C G
A A U
Another tRNA molecule brings the next amino acid into place.
A U G G G C U U A A A G C A G U G C A C G U U

24. C C G
C C G
A peptide bond joins the second and third amino acids to form a polypeptide chain.
A U G G G C U U A A A G C A G U G C A C G U U

25. A C G
G U C
The process continues.
The polypeptide chain gets longer.
This continues until a termination (stop) codon is reached.
The polypeptide is then complete.
A U G G G C U U A A A G C A G U G C A C G U U

26. Rules of Translation The start codon is AUG which codes for methionine
What does this mean about the first amino acid in every protein?
Chain elongation continues on the ribosome as it reads the mRNA strand
Translation continues until a stop codon is reached
The stop codons, unlike the start codon, do not encode amino acids
Completed protein is then folded with help from chaperones

27. But what happens when this process goes wrong?

28. Mutations
Frame shift mutation: adding or deleting one base causes a change in the reading frame…why?

29. Missense mutation: a base change that results in substituting one amino acid for another

30. Nonsense mutation: a base change that results in substituting a stop codon in place of an amino acid
This results in early termination of the protein

31. Silent mutation: a base change that results in no change in the encoded amino acid (or stop codon)
Why could this happen?
Are these still dangerous? Why or why not?
REDUNDANCY!
Not really – still makes the same exact protein product